Signs of "latent radicalisation" have been detected in the attacker who knifed four co-workers to death at police headquarters in Paris, France's anti-terrorism prosecutor says.

In the video above: Paris police killer followed radical Islam

An IT worker at the headquarters went on a seven-minute rampage on Thursday, killing three police officers and an administrative worker, and wounding at least one other, before being shot dead by police.

Anti-terrorism prosecutor Francois Ricard said his office had taken over the probe because of signs the crime was premeditated and because of the nature of injuries found on at least one of the victims.

The prosecutor also said the attacker had stated a desire to die.

"The context of latent radicalisation" and messages of exclusively religious character the attacker sent to his wife shortly before the crime were added factors, Ricard told a news conference on Saturday.

The investigation also revealed contacts between the attacker and several individuals who are likely to belong to an Islamist Salafist movement, Ricard said.

Ricard said the autopsies "attest to a scene of extreme violence" in an attack that lasted seven minutes, adding that two knives used had been purchased by the killer.

The man "agreed with certain atrocities committed in the name of (Islam)" and defended the Charlie Hebdo attacks in 2015, Ricard said.

The suspect, who was shot dead during the attack, exchanged 33 text messages with his wife ahead of the attacks, all of which were of a religious character, Ricard said, adding that the suspect's wife is in police custody until Monday.

Officials have not officially said there was a terrorist motive behind the attack, but handing a case to anti-terrorism prosecutors usually indicates a terrorism link is the focus of inquiries.

The killer, 45, has been identified by officials only as Mickael H.

Ricard said that during a "deadly journey" the attacker first stabbed two police officers.

A third police officer was killed in another office and an administrative worker died on the stairs.

The attacker was born on the French island of Martinique and had worked at the police headquarters for several years. He converted to Islam about 10 years ago, Ricard said.